External-cavity quantum cascade lasers (EC-QCLs) have received considerable attention in the academic literature because a single EC-QCL can be tuned over a significant portion of the ‘fingerprint region’ of the electromagnetic spectrum. This makes EC-QCLs potentially useful in a wide range of chemical detection applications. An EC-QCL typically includes an external optical cavity in which are located an optical tuning device, such as a grating, and a QCL chip that generates light. The QCL chip includes a periodic series of thin semiconductor layers of varying material composition that form a superlattice in which a single electron can cause the emission of multiple photons.
EC-QCLs typically have to be operated in a pulsed mode (sometimes referred to as a “quasi-continuous wave mode” or a “quasi-CW mode”) to achieve broad tunability. In pulsed mode, the QCL chip is switched on for brief time period (typically 50-500 ns, and referred to herein as an ON period), and then switched off for a much longer time period (typically 0.1-1 μs, and referred to herein, and referred to herein as an OFF period) to allow the QCL chip to cool down before the next ON period. Operated this way, the QCL chip never achieves thermal equilibrium. Additionally, the temperature of the QCL chip increases between the beginning and the end of each ON period. The increasing temperature causes the frequency of the light generated by the QCL chip to change monotonically between the beginning and the end of each ON period. This change in frequency is known as chirp. In an EC-QCL, the changing frequency of the light generated by the QCL chip causes the EC-QCL to hop among the longitudinal modes of the external cavity. When a mode hop occurs the frequency, phase, and intensity of the light emitted by the EC-QCL all change discontinuously. There may be as many as six or more mode hops over the duration of a single 200 ns pulse. Mode hops increase the intensity noise in an EC-QCL, and make the frequency and phase of the emitted light unstable.
In some applications it is desirable to average pulse data sets representing multiple pulses of light generated by a light source to reduce the effect of noise. However, in applications in which an EC-QCL operated in quasi-CW mode or another type of pulsed laser that exhibits mode hopping is used as the light source, averaging and some other post-processing techniques are problematic.
Accordingly, what is needed is a way to use averaging and other post-processing techniques in applications in which a pulsed laser that exhibits mode hopping, such as an EC-QCL, is used as the light source for performing optical measurements.